Such devices are known and document WO 2005/000627 shows such a device. In the known device during collision inertia forces on the child tension the belt and the upper slide pulls on the upper part of the support back. As the support back is made of plastic material that is flexible, the upper part will deform due to the forces and this will cause additional movement of the child under the inertia forces. The additional movement of the child causes the risk of injury to the child to increase and this must be avoided.